This invention relates generally to speed sensing devices. More particularly, this invention relates to a speed sensing device for determinig the speed of a rotating object, for example, a shaft, utilizing an optical system operable to determine the rotational speed from a diffuse reflective spectra where no reflective singularity exists and not requiring the use of a reflective element on the rotating object.
Customarily, when using an optical-type tachometer, it is necessary to apply a piece of retro-reflective tape or paint to a rotating machine element to provide a reflective target. When illuminated by a beam of light, the reflective target provides a distinct reflective signal which may be detected and processed to determine the rotational speed of the machine element. Alternatively, certain tachometer techniques utilize an area of a shaft where a single discontinuity exists, such as a flat or keyway. Reflections can be specular (mirror-like), diffuse (scattered), or a combination of the two.
A retro-reflector is a special case, wherein the reflected beam of light returns along a path incident to the illuminating light beam. Normally, for retro-reflective targets, the reflected beam exhibits only a small amount of divergence. Correspondingly, retro-reflective techniques provide for the maximum light return from the target. However, it is not always convenient or possible to affix a retro-reflective target to a machine element, since the machine may be operating or difficult to access. Moreover, the machine elements may be coated with oil or other fluids making adhesion of a reflector difficult, for example, a felt conveyor belt as used in a paper mill. Often it is necessary to use the trademark or other symbols which are dyed into a fabric as a target for an optical tachometer. Typically, these target types provide a very diffuse reflection, severely limiting the operational distance of optical-type speed sensors. Additionally, debris and moisture accumulating on the optics at close operating distances makes a longer operating distance desirable.
Optical tachometers utilize a single reflective discontinuity per revolution of a machine element to develop a single electrical pulse. The accuracy of the optical tachometer is based on the electrical pulse occurring at the same physical point on the machine element each revolution. This fundamental principle is necessary for the accurate measurement of the time between pulses for determining the rotational speed of the machine element. Moreover, it is also important for determining the phase relationship of frequencies within a vibration signal when monitoring the vibration spectrum of a machine. As an example, it is often desirable to know the phase and amplitude of the vibration component at one times running speed (1xc3x97revolutions per minute.). This phase measurement, locked to some physical orientation of a shaft, is fundamental to balancing machines and determining operating deflection shapes.
What is needed, therefore, is a speed sensor that does not require the application of a reflective target and is operable to determine the speed of a rotating object, such as a rotating shaft.
The optical speed sensing system according to the invention advantageously provides speed measurements where it is difficult or impossible to obtain a distinct reflected light pulse from a rotating object. This is particularly advantageous in portable route based applications. Additionally, the system is operable to provide a convenient determination of the running speeds of a large number of operating machines having no reference marks or difficult to access marks. Surface irregularities of an object provide an intensity-modulated reflection when the rotating object is illuminated by the coherent light source. The same light intensity patterns are repeated once per revolution of the rotating object.
An apparatus and method are provided for determining the rotational speed of a rotating object, without requiring the location of a reflective target on the object. The apparatus includes a light source for transmitting a collimated light beam having a selected wavelength directly at the rotating object. Light signals are reflected from the rotating object, due to the transmission of light at the object. An optical system focuses the reflected light signals onto a single detection region of a detector. The detector is operable to detect the focused light signals and generate electrical signals based on the signals reflected and focused by the optical system. An analyzer analyzes the electrical signals representative of the reflected light signals. The analyzer includes a Fourier Transform function for transforming the electrical signals providing spectra representative of the electrical signals. The analyzer operates to determine and output the rotational speed of the rotating object based thereon. A power source is included for providing power to the apparatus.
A manual and an automated method are available to users of the apparatus in determining the speed of a rotating object.